S3·E15 Yesterday's Enterprise

Stardate: 43625.2. A rift in the space/time continuum brings the U.S.S. Enterprise - C forward into the 24th Century, and alters the timeline. In this future, the Federation is at war with the Klingons, Tasha Yar is on the bridge, Worf did not join the Federation, and only Guinan's unique perception of reality may restore the timeline.